Volume 13, Issue 3 -

This paper presents an extensive review of the kinetics, hydrodynamics, mass transfer, heat transfer and mathematical as well as computational fluid dynamics (CFD) modeling of Low-Temperature Tropsch Synthesis (LTFT) synthesis in Slurry Bubble Column Reactors (SBCRs), with the aim of identifying potential research and development areas in this particular field. The kinetic expressions developed for F-T synthesis over iron and cobalt catalysts along with the water gas shift (WGS) reactions are summarized and compared. The experimental data and empirical correlations to predict the hydrodynamics (gas holdup, Sauter mean bubble diameter, and bubble rise velocity), mass transfer coefficients and heat transfer coefficients are presented. The effects of various operating variables, including pressure, temperature, gas velocity, catalyst concentration, reactor geometry, and reactor internals on the hydrodynamic and transport parameters as well as the performance of SBCRs are discussed. Additionally, modeling efforts of SBCRs, using axial dispersion models (ADM), multiple cell recirculation models (MCCM) and computational fluid dynamics (CFD), are addressed. This review revealed the following: (1) Numerous F-T and WGS kinetic rate expressions are available for cobalt and iron catalysts and one must be careful in selecting the appropriate expressions for LTFT. Iron catalyst suffers from severe attrition and subsequent deactivation in SBCRs and accordingly building a costly catalyst manufacturing facility onsite is required to maintain a steady operation of the F-T reactor; (2) Experimental data on the hydrodynamic and transport parameters at high pressures and temperatures, typical to those of actual F-T synthesis, remain scanty when compared with the plethora of studies conducted using air–water systems in small reactors at ambient conditions; (3) Several empirical correlations for predicting the hydrodynamic and mass as well heat transfer parameters are available and one should select those which consider the reactor diameter, gas mixtures and the potential foamability of the F-T liquids; (4) The effect of cooling internals configuration and sparger design on the hydrodynamic and transport parameters, local turbulence, mixing and catalyst attrition are yet to be seriously addressed; (5) The impact of operating variables on the hydrodynamic and transport parameters as well as the overall performance of the SBCRs should be investigated using actual F-T fluid–solid systems under typical pressures and temperatures using a large-scale reactor (>0.15 m ID) in the presence of gas spargers and cooling internals; (6) Significant efforts are still required in order to advance CFD modeling of SBCRs, particularly those pertaining to the relevant closure models, such as drag, lift and turbulence. Also, cooling internals configuration and the design as well as orientation of gas spargers should be accounted for in the CFD modeling; and (7) Proper validations of the CFD formulations using actual systems for F-T SBCR are needed.

A careful review of the literature reveals that extensive research has been done on the hydrodynamics in packed bed columns using turbulence models. It can be noted that the choice of turbulence model is influenced by the number of phases, type of fluid, Reynolds number range and the type of packing. Thus, comparison of turbulence models for the selection of a suitable model assumes great importance for the better prediction of flow pattern. This is due to the fact that poor prediction of the flow pattern can lead to a limited heat and mass transfer model as the rate of transfer processes in packed bed is governed by the hydrodynamics of the packed bed. The aim of this paper is to give a review of the computational fluid dynamics (CFD)-based hydrodynamics studies of packed bed columns with the primary interest of studying pressure drop and drag coefficient in packed beds. From the literature survey in Science Direct database, more than 48,000 papers related to packed bed columns have been published with more than 3,000 papers focused on the hydrodynamic studies of the bed to date. Unfortunately, there are only a few studies reported on the hydrodynamics of packed columns under supercritical fluid condition. Therefore, it is imperative that the future work has to focus on the hydrodynamics of supercritical packed column and particularly on the selection of suitable turbulence model.

The staggered herringbone chaotic micromixer has been designed based on the use of modular method and orthogonal experiment. With the modular method, the geometry of the micromixer was divided into straight channels and mixing units alternately. The mixing units were designed with orthogonal experiment. The aspect ratio of the herringbone to the microchannel (r1), the ratio of the width of herringbone to the spacing of between adjacent two herringbones (r2) and the width of the herringbone (r3) were investigated. The optimal outputs were r1 = 1:3, r2 = 1:1, and r3 = 50 µm. The micromixer was fabricated with two steps lithography method based on the presented optimal parameters, and the material of the micromixer was polydimethylsiloxane (PDMS). The optimized design method is proven to be an effective way for rapid design of the staggered herringbone chaotic micromixer.

Effects of the types and addition amounts of sludge on the true rheological properties of petroleum coke water/sludge slurry (PCWS/PCSS) flowing in pipelines were investigated using the Tikhonov regularization method. Results show that PCWS of 59.8 wt% changes from pseudo-plastic fluid to dilatant fluid as the shear rate increases. However, PCWS of 63.4 wt% is a very complex fluid: shear thinning at low shear rate, followed by shear thickening over a critical shear rate, and a subsequent shear thinning at high shear rate. Rheological properties of PCWS have a significant change after the sludge was added. PCSS changes from dilatant fluid to pseudo-plastic fluid when the addition amounts of sewage sludge ascend to 10 wt%. Petroleum coke particles are trapped by the stable “network” structures, which are formed by flocculent sludge particles. The wall slip velocity of PCSS is higher than that of PCWS with the sludge amount increases, which is beneficial for pipe transportation. In addition, the wall slip velocity of PCSS containing sewage or petrochemical sludge increases with the wall shear rate increases, but the slip velocity of PCSS containing paper mill sludge first increases and then decreases.

In the present work, crude bio-oil obtained from vacuum pyrolysis of babul wood was stabilized by esterification with 1-butanol using the cation exchange resin, Amberlyst-15, as a solid acid catalyst. Ester formation reduces the pH, thereby increasing the shelf-life of the bio-oil. Since esterification is a reversible reaction, simultaneous separation of water during the course of the reaction helps to obtain high conversion. Azeotropic removal of water by reactive distillation was found to be effective in this work. Apart from reducing pH and improving shelf-life, this process also enabled water removal from crude bio-oil, and viscosity was reduced when the bio-oil was blended with alcohol. Amberlyst-15 was found to get deactivated after repeated use. Characterization of fresh and used catalyst by Brunauer–Emmett–Teller (BET) surface area measurement and thermal analysis showed that the deposition of carbonaceous material on the catalyst is responsible for its deactivation. The condensation and oligomerization reactions of unstable compounds (e.g. furfural and its derivatives) are suspected to be the main reasons underlying catalyst deactivation.

A pre-combustion removal option, coupling water–gas shift and CO 2 capture is the well-known sorption-enhanced water–gas shift (SEWGS): the removal of CO 2 produced by WGS reaction, shifting the thermodynamic equilibrium, enhances H 2 production. Among the different CO 2 sorbents, hydrotalcite-like compounds work at the required intermediate temperature ( T = 200–400°C). Using low supersaturation method, three different sorbents were synthesized. Sieved fractions were impregnated with 20% w/w K 2 CO 3 and then dried and subjected to thermal treatment. Sample characterization was performed by means of FT-IR spectroscopy, XRD analysis and TG-DTA analysis. Sample analysis was carried out after synthesis, thermal treatment (calcination) and after fixed bed reactor capture tests. Sorption and desorption tests were performed in a fixed bed microreactor, under cyclic conditions, at temperature level of T = 350°C and P = 5 bar in dry and wet condition. The amount of CO 2 captured by the sorbent in each test was quantified by means of a first order with dead time flow distribution model applied to the experimental system. Sorption capacity of sorbents in dry conditions increases of 30% with respect to previous atmospheric pressure results obtained in fluidized bed. These sorbents seem to be good candidates to be used as a bi-functional sorbent-catalyst for SEWGS.

A spouting bed reactor using rice straw as feedstock for further gasification purposes was studied in the present work. Experimental fluid dynamic tests were carried out in a conical square-based spouted bed reactor using a mixture of straw and silica, the latter acting as inert material. The influence of the initial bed of particles and the presence of coarse particles in the mixture on the pressure drop and minimum spouting velocity were evaluated. As a result, lower values of pressure drop along the bed were found compared to conventional fluidisation processes. Higher initial bed heights and higher percentage of straw in the mixture resulted in higher values of pressure drop and minimum spouting velocity. A new correlation for a cross section spouted bed reactor was proposed for the prediction of the minimum spouting velocity.

In recent years, great efforts have been directed to provide eco-friendly methods for nanoparticles (NPs) synthesis. In this endeavor, it is desired that polydispersity be as narrow as possible and that the chemical and physical properties can be controlled. In this work, silver nanoparticles (SNPs) were obtained by means of (a) a green approach (biosynthesis) using alfalfa extracts; and (b) a thermal decomposition method in organic media. As per biosynthesis, pH, initial concentration of precursor (Ag + ) and extraction solvent of plant metabolites were varied in order to identify the conditions where SNP polydispersity presented a best value. When these conditions were determined, the reaction kinetics was evaluated. The rate constant and order of reaction were 7.33×10 −6 L 3.6 /mol 3.6 s, and 4.6, respectively. Also, in the biosynthesis, it was found that the size and the degree of polydispersity depend on initial concentration of precursor and the type of extractant. Thermal decomposition was performed using silver oleate as precursor in order to compare characteristics of the NPs obtained by both biosynthesis and the chemical method. According to our results, SNPs obtained through thermal decomposition showed a lower polydispersity and higher degree of crystallinity than those obtained using biosynthesis. However, the green method eliminates the use of toxic compounds, which is extremely important if these particles are intended for biomedical purposes. In addition, this is a less expensive method as compared to other chemical methods. To our knowledge, this is one of the few reports analyzing the reaction kinetics, which is extremely important if scale-up is intended.

Despite the hydrodynamics of trickle beds experiencing pilot size, on the contrary, the possibility of measuring hydrodynamic parameters locally and for large scale and finding an indicator to pinpoint the flow regime from trickling to pulsing remains obscure, to assess the feasibility of getting such information the reactor bed was randomly packed with equal-sized spherical beads of 3 mm diameter. Gas and liquid holdup measurements and flow regime identification were achieved at various flow conditions from trickling to the pulsing regime by an optical probe method using time series processing method. This method is shown to be simple and efficient provides quantitative information about the gas and liquid holdup variation in different flow regime. The various experimental results were compared to available literature models and correlations for confrontation and recommendations. The trickle-to-pulse flow transition boundary shifted towards higher gas and liquid superficial velocities, aligning with the findings on a variation of holdup along the axial direction confirmed to broaden the trickle flow domain. [Correction added after online publication June 26, 2015: To unify the adjective everywhere in this manuscript, optic fiber probe was changed to optical probe.]

A combined process of solvent extraction and two-phase biodegradation was carried out to remove crude oil from water by mixed cultures, where silicone oil was selected as the organic solvent due to its biocompatibility and non-biodegradability. The crude oil removal and cell growth was experimentally studied. A simple model that combined steady mass transfer equations and dynamic growth kinetics of suspended cells was suggested to follow the entire process. Under the conditions studied, complete removal of crude oil from water was achieved at initial crude oil concentration of 5,000 mg/L. Results revealed that the proposed model satisfactorily described the process as long as crude oil level in the cell medium did not exceed the toxicity limit of suspended cells.

Within the framework of green chemistry, catalysts should be met different criteria such as biodegradability, recyclability, flammability, non-toxicity and low price. Acidic deep eutectic solvent (DES) have been synthesized for this purpose, by mixing para-toluene sulfonic acid and choline chloride. The catalytic activity of DES was studied in the esterification of acrylic acid with n-butanol. The usage of DES as catalyst is simple, safe and cheap. The effects of temperature, catalyst loading, n-butanol/acrylic acid molar ratio on the conversion of acrylic acid were performed. The batch reactor experiments were carried out at temperatures of 338, 348, 358 and 368 K, molar ratio of butanol to acrylic acid of 1, 2,3 and catalyst loading of 10, 15, 20 and 90 g/L. 90.2% of acrylic acid conversion was achieved at a temperature of 358 K and catalyst loading of 20 g/L. Reusability of DES was investigated. Reusability and catalytic activity makes DES efficient as catalyst.

Jatropha curcas oil and methanol are transesterified using potassium triphosphate as base catalyst. The effects of methanol to oil molar ratio, reaction temperature, stirring speed, catalyst concentration, solubility and its reusability on the yield of biodiesel are investigated. The base catalyst tri-potassium phosphate (K 3 PO 4 ) is found to be highly suitable for oils having less than 1.5% free fatty acids (FFA). Highest biodiesel yield (approximately 92%) is acquired under optimum conditions of 9:1 methanol to oil molar ratio, 2% catalyst at 70°C reaction temperature at a stirring speed of 650 rpm. The chemical activity of K 3 PO 4 is found to be similar to that of base catalyst potassium hydroxide (KOH) and the catalyst solubility in biodiesel as determined by atomic absorption spectra is only 4.81 ppm. It has been found that K 3 PO 4 is highly hygroscopic and its reusability drastically decreases upon further usage and it can be reused only in wetted condition for three continuous usages with drastic reduction in catalytic strength. The biodiesel samples prepared were tested for several physicochemical properties and compared with the values of European biodiesel standards. The fatty acid methyl esters (FAME), also referred to as jatropha methyl esters (JME) in this paper, have been analyzed by gas chromatography and thermogravimetric analysis.

Ethyl lactate is widely used as a solvent for several industries and usually produced by esterifaciton of lactic acid with ethanol. In order to lower its production cost, attempt to synthesis this ester directly from lactate salt produced by bacterial fermentation was made. In this work, reactive distillation process for small-scale synthesis of ethyl lactate from esterification of fermentation-derived magnesium lactate with ethanol was designed and simulated using Aspen Plus simulator equipped with RADFRAC module. Non-ideality of the reaction components was represented by UNIQUAC property model. Specification and optimum conditions of reactive distillation and fractional distillation units in the proposed process scheme were obtained. Under the process scheme and operating conditions proposed in this work, approximately 96.13% of lactic acid was converted and 95.76% yield of ethyl lactate was achieved.

The oxidation of m-xylene(MX) to isophthalic acid(IPA) catalyzed by phosphotungstic acid(HPW) supported on modified activated carbon was investigated. The activated carbon loading with HPW is modified by HNO 3 solution to ameliorate its catalytic capability in the oxidation of MX to IPA. Experiments have been carried out to study the effects of modification conditions, such as HNO 3 concentration, impregnation time, impregnation temperature, activation temperature and activation time, on the catalytic performance of activated carbon. The experimental results demonstrate that the carbon sample impregnated in 10%(vol) HNO 3 solution at 45°C for 8 h followed by calcined at 700°C for 4 h has the best catalytic capability. The characterization results imply that the specific surface area and micropores of the carbon samples decrease after being treated with HNO 3 solution. But the acidic functional groups on the activated carbon surface increase, which play a vital role in improving the catalytic ability of the HPW/C catalyst in the oxidization of m-xylene to IPA.

Using a 20-run central composite design, standard Basal Salt Media (BSM) was optimized for the production of recombinant phytase by Pichia pastoris in bioreactor-based fed-batch cultivations using glucose as the carbon source. The phytase activity in the supernatant of the cultures at the end of 48-hour runs was modeled as a function of the medium composition, and this model was successfully validated. Using Inductively Coupled Plasma spectroscopy (ICP), residual elements were quantified in the cultivation broth at the end of each run to confirm that differences in final cell density and enzyme activity were not due to nutrient depletion, but indeed associated with the variations in medium composition. The optimized media contains significantly lower concentration of the potassium and magnesium sources (28.7 g/l KH 2 PO 4 and 4.8 g/l MgSO 4 ·7H 2 O respectively), as well as a reduced concentration of trace salts, and based on experimental results, significantly outperforms standard media in terms of recombinant product yields under the same conditions.

Present study reports parametric and kinetic study for catalytic per oxidation (CPO) of pyrrole by Cu/SBA-15 catalyst using H 2 O 2 as an oxidant. Brunauer-Emmett-Teller surface area, Fourier transform infra-red spectroscopy and thermo gravimetric-differential thermal analyses were used for the characterization of catalyst. Effects of various operating variables such as initial concentration of pyrrole (C o ): 48.3–386.8 mg/L, catalytic dose (C w ): 0.5–2 g/L, stoichiometric ratio of hydrogen peroxide/pyrrole: 1–4, and reaction temperature (T): 50–60°C were studied. More than 85% pyrrole mineralization was observed at the optimum conditions of C w = 1.5 g/L, stoichiometric ratio of hydrogen peroxide/pyrrole = 3, T = 55°C at pH = 6.1. A two-step pseudo-first-order kinetic model well-described the pyrrole mineralization by the CPO process.